Surgical microscopes are used, for example, as an aid in operations on the human eye, for instance in laser treatment of the eye in order to ablate corneal tissue or to make cuts in the cornea or other parts of the eye. So that the operator has his hands free during the operation and to keep the operator's hand from being contaminated by touching actuation elements of the microscope or other controllable components used during the operation, it is known in the prior art to overlay an image into the observation beam path of the surgical microscope, which represents a control menu on which the operator (observer) can visually (i.e. by controlled viewing) select and activate individual control functions. To this end it is necessary to detect the observer's viewing direction, for which purpose in the prior art a camera is provided, for example having a CCD sensor. A beam is in this case extracted from the observation beam path of the microscope and directed onto the camera. Using suitable image analysis algorithms, the details of which are not of further interest in the scope of the invention, it is possible to determine the observer's viewing direction relative to the overlaid control menu. Depending on where the observer is looking at on the overlaid control menu, it is possible for the observer to activate different functions.
By way of example, in respect of the prior art relating to surgical microscopes which are equipped with devices for overlaying a control menu and for detecting the direction in which an observer looks at the overlaid control menu, reference may be made to WO 96/13743 A1. There, the control menu is displayed on an LCD screen and overlaid into the observation beam path by means of suitable optical elements. In order to record the pupil position of the observer's eye, a CCD camera is provided.